1. Field of the Invention
The present invention relates to spatial light modulators (SLM) for amplitude and phase modulation of incident light and, more particularly, to a mirror and to a hinge mechanism for miniaturized spatial light modulators.
2. Description of the Prior Art
Microdevices for light modulation go back 20 years or more. In the majority of cases, these devices are comprised of an electrostatically actuated mirror, supported by torsion bars along the axis of rotation of the mirror. A classic example can be found in U.S. Pat. No. 4,317,611 issued on Mar. 2, 1982 to Petersen, in which an “Optical Ray Deflection Apparatus” is described.
The following United States Patents and Patent Application Publications are also known.
U.S. Pat. No.Issue DateInventor(s)3,886,310May 27, 1975Guldberg et al.4,615,595Oct. 7, 1986Hornbeck4,662,746May 5, 1987Hornbeck5,061,049Oct. 29, 1991Hornbeck5,083,857Jan. 28, 1992Hornbeck5,096,279Mar. 17, 1992Hornbeck et al.5,212,582May 18, 1993Nelson5,216,537Jun. 1, 1993Hornbeck5,233,456Aug. 3, 1993Nelson5,293,511Mar. 8, 1994Poradish et al.5,312,513May 17, 1994Florence et al.5,331,454Jul. 19, 1994Hornbeck5,382,961Jan. 17, 1995Gale, Jr.5,392,151Feb. 21, 1995Nelson5,444,566Aug. 22, 1995Gale et al.6,025,951Feb. 15, 2000Swart et al.6,147,789Nov. 14, 2000Gelbart6,268,948Jul. 31, 2001GelbartU.S. Publ. No.Publ. DateInventor(s)2002/0135857A1Sep. 26, 2002Fitzpatrick et al.2002/0150377A1Oct. 17, 2002Gelbart2002/0154380A1Oct. 24, 2002Gelbart
As mentioned hereinabove, microdevices for light modulation have been around for many years. In the majority of cases, these devices comprised an electrostatically actuated mirror, supported by torsion bars along the axis of rotation, as in aforementioned U.S. Pat. No. 4,317,611 to Petersen describing an “Optical Ray Deflection Apparatus”.
Before proceeding with a detailed description of the present invention, it is worthwhile to briefly discuss the basic fundamentals of electrostatic light modulating devices, including an overview of SLM devices.
In FIG. 1a, schematics of the three fundamental SLM architectures and associated modulation properties are shown. To the left, a torsion mirror 10 which provides amplitude modulation is suspended by torsion hinges 12 along an axis of symmetry, e.g., an axis that extends through the middle of opposed edges of a square mirror, an axis that extends through opposed corners again of a square mirror, etc. The torsion mirror 10 can be used to steer light into, or away from, the acceptance cone of an optical imaging system. Amplitude modulation is obtained by controlling the length of time the light is directed into the acceptance cone during one image frame. The technique is referred to as pulse-width modulation. This type of SLM is typically operated in a digital mode.
In the middle of FIG. 1a, a cantilever type SLM 14 is supported by either a torsion hinge 16 or a flap-based hinge 18. This type of SLM provides both amplitude and phase modulation, and is typically operated in an analog mode.
A piston type SLM 20, shown on the right of FIG. 1a, provides strictly phase modulation of the incident light. The piston type SLM 20 is usually supported by four flap hinges 22 and the mirror moves in a direction perpendicular to its plane. This device is usually operated in analog mode.
Of these three SLM designs described hereinabove, the torsion based device is the most common, in part due to its popularization by Texas Instruments. The discussion which follows will therefore focus primarily on this type of device. Nevertheless, the underlying principles, can for the most part, be applied to the other SLM designs.
In FIGS. 1b and 1c, generic schematics of an electrostatically actuated torsion hinge device 24 are shown. FIG. 1b illustrates the basic architecture, while FIG. 1c represents a schematic vertical cross section of the device 24. In FIG. 1b, it is shown that the device architecture includes a mirror 26 suspended along an axis of rotation 28 by torsion bar hinges 30 (as discussed earlier), the axis of rotation in this particular device 24 extending symmetrically through opposed corners of the mirror 26. The torsion bars 30 are in turn suspended above an underlying substrate 32 by support posts 34. Also on the substrate 32, some distance from the axis of rotation 28, are drive electrodes 36.
The mirror 26 and drive electrodes 36 are electrically conductive, and form a parallel plate capacitor.
Applying a potential difference between the mirror 26 and one of the drive electrodes 36 loads charge onto the capacitor plates, the quantity of charge being a function of the effective capacitance and the potential difference. Because the charge on the mirror 26 and given electrode 36 are of opposite sign, an electrostatic attractive force is generated which pulls the mirror 26 towards the charged electrode 36. This, in turn, causes the mirror 26 to tilt an angle θ about its axis of rotation 28. The maximum angle of tilt, θ=θmax, occurs when the mirror 26 touches the underlying substrate 32, and is governed by the maximum length of extension of the mirror 26 from its axis of rotation 28, and the height of the support posts 34. Typical values for θmax range from a fraction of a degree to as much as 10°. It is common practice to define a normalized tilt angle α, viz. α=θ/θmax.
A number of modes of operation for such a torsion hinge device 24 are possible, and they include bistable, tristable, and analog. In bistable mode (binary mode), the mirror 26 is operated in only two states, namely α±1. Here, a sufficiently high potential is applied to a given address electrode 36 so that the mirror 26 turns until it rests on the underlying substrate 32. This is a useful mode of operation when the application requires binary amplitude modulation of an incident light beam. In the second mode of operation, the mirror 26 has three states, α=0°, α±1. While this mode of operation is in principle possible, it is not often used because the α=0° position is usually imprecise; because of mechanical and material considerations, it is not guaranteed that the torsion hinges 30 will restore the mirror 26 to precisely the α=0° position. In the third mode of operation, a variable voltage is applied to the address electrodes 36, causing the mirror 26 to tilt in an analog fashion. The theory behind these three operational modes is rather involved, and is elaborated in more detail in several of the patents cited herein.
Cantilever and piston type devices are generally operated only in analog mode. The operation is similar to that described above for analog operation of the torsion type device 24. Note that in the cantilever and piston type devices there is typically only one address electrode.
Certain requirements must be met in order for a SLM device to be useful. For one, the torque required to twist the torsion bars (restoring torque) must not exceed the torque that can be generated electrostatically using a “reasonable” potential difference. The acceptable maximum potential difference is typically governed by the type of control circuitry used, although other factors can prevail as well.
The mirror shown in FIG. 1a must be rigid enough that it does not bend significantly when actuated. Bending occurs because the force that causes the mirror to move is not matched by an equal and opposite force on the other side of the axis of rotation. The acceptable level of bending is dictated primarily by the light wavelength(s) for which the device is intended to operate, and the optical configuration in which it is incorporated.
The mirror must be spaced sufficiently high enough above the underlying substrate so that an adequate angle of tilt is provided for the given application. Since large vertical distances are difficult to obtain using thin film processing technologies, tilt angles are generally limited to small values. Assuming a flat mirror and substrate, then θmax=Tan−1(h/l) where h is the height of the supports, and l is the length of extent of the mirror taken perpendicular to the axis of rotation.
Finally, a high fill factor is important, particularly when SLM's are arranged in arrays. Therefore, the area required for the support mechanism must be small relative to the mirror area. Because of processing considerations, high fill factors become more difficult to obtain as mirror size is reduced.
Similar requirements exist for the cantilever and piston type devices.
One of the first examples of such a SLM device, or more precisely, a μ(micro)-SLM device is found in U.S. Pat. No. 3,886,310 to Guldberg et al., which was issued in 1975. A brief discussion of the invention thereof is worthwhile as it illustrates some of the underlying concepts of μ-SLM's. The device is used to project images by diffraction, using a complex optical system, which includes the usual Schlieran stop (or equivalent) found in most diffraction based projection systems. Electrostatic actuation is achieved by charging the planar reflective portion of the structure with an electron beam (the mirror array is mounted inside an electron picture tube). This device is intended primarily for projection in the visible part of the spectrum. The hinge mechanism is a type of flap connected to one corner of the mirror, which bends when the mirror is tilted. The mirrors are arranged in a cloverleaf fashion, with four hinged mirrors (wings) supported by a central post.
Petersen's aforementioned U.S. Pat. No. 4,317,611 is directed to a device that represents a key milestone in the development of miniaturized SLM's. It is constructed in a hybrid fashion, with the lower portion containing the address electrodes, and the upper portion containing the mirror. These two portions are glued together. It is noted that this construction makes mass production of the device rather difficult.
Electrostatic actuation is achieved by applying a positive bias to one of the address electrodes with respect to the mirror, causing the mirror to tilt about it. It is worth noting that Petersen states that the actuation voltage is in the order of 300 volts, significantly higher than what can be obtained with standard IC semiconductor circuitry. The supporting “land” prevents the mirror from collapsing on the substrate when actuated due to the long torsion hinges. The intended application for this device is primarily in optical scanning systems, in which the mirror is operated at, or near, a mechanical resonance.
Subsequent to the Petersen Patent, an extensive collection of developments were made in the art of SLM's by Texas Instruments (TI) Inc., a recognized leader in the μ-SLM field. Texas Instruments has made extensive developments over the years to their μ-SLM technology. Note that TI uses the acronym DMD when referring to their technology, which originally stood for “Deformable Micromirror Device”. TI has since changed the meaning of the acronym to “Digital Micromirror Device”.
Texas Instruments have geared their DMD technology towards two key applications: high-resolution projection displays, and high-resolution colour printing systems. The requirements associated with these applications have therefore had a significant influence on TI's DMD architecture. The mirrors are small (16 μm by 16 μm) and tightly spaced (17 μm pitch), so that large density arrays can be fabricated in a manageable area of silicon. For example, a 1000 by 1000 pixel array occupies only 2.9 cm2 of silicon. The mirrors also switch very quickly (≈5 μs), a necessary requirement because a pulse width modulation scheme is used to achieve grey scale imaging; the faster the mirror switching time, the larger the number of intensity levels that can be produced. The DMD arrays are manufactured on top of a standard 5V CMOS circuitry and therefore have low switching voltages. The 5V CMOS technology is mandated by the extremely high mirror density requirements.
The first TI patent in this area, U.S. Pat. No. 4,615,595, is entitled “Frame Addressed Spatial Light Modulator” and issued in 1986. This Patent describes a cloverleaf mirror design, fabricated on a CCD CMOS circuit.
The underlying CCD is used to pump electrical charge onto the cloverleaf, causing all four mirrors to deflect downward to the ground plane. While the method of charging the mirror is novel, this device is very similar in concept to that described in U.S. Pat. No. 3,886,310 mentioned previously. In particular, the cloverleaf and hinge mechanism is for all intents and purposes the same as that described by Guldberg and Nathanson.
The architecture and control circuit approach disclosed in U.S. Pat. No. 4,615,595 has since been replaced by another design which will be addressed hereinafter. One reason was undoubtedly due to the poor contrast of projection systems employing the device, since its operation requires a Schlieran configuration (or equivalent). In U.S. Pat. No. 5,061,049 awarded to Hornbeck in 1991, the mirror is supported by torsion hinges that twist when the mirror rotates. In this regard, the design is similar to that of Petersen (U.S. Pat. No. 4,317,611), although here the manufacturing process is monolithic. Also, this mirror design does not require a Schlieran projector for imaging, and can therefore be used to produce high-brightness, high-contrast images.
The torsion hinges must clearly be made thinner than the mirror in order for the device to rotate without warping. The hinges are supported above the substrate by a spacer layer (or equivalent, such as metal support posts). Electrostatic actuation is achieved using the addressing electrodes. In order that the mirror can be turned with small voltages, these hinges must be extremely thin, ˜700 angstroms, and fairly long, i.e. more than 8 μm. While the torsion hinge design is suitable for small mirrors, it does not work well for large mirrors; larger structures will tend to sag in the middle if support is provided only at their edges. In addition, because the hinges must be thin, and long, they generally exhibit a low thermal conductance. This reduces the mirrors capability to adequately dissipate heat, which can result in irreversible damage should it be employed for modulation of high energy beams.
Using the fabrication process described in this same U.S. Pat. No. 5,061,049, a variety of hinge configurations are possible. For example, the mirror can be hinged at one corner, or along one edge. It must be noted that when the mirror is hinged at a corner, the hinge is of the bending type, rather than the torsion type.
U.S. Pat. No. 5,083,857 issued to Hornbeck in 1992 describes a novel two level mirror structure in which the torsion hinges are placed in a layer beneath the mirror layer. This two-layer structure was required to improve of the low fill factor of TI's first generation mirror design. As was mentioned earlier, torsion hinges need to be fairly long for sufficient flexibility thereby limiting the packing density of mirrors supported by such hinges. For the small mirrors, the fill factor is inadequate using a one-level architecture. By placing the hinge mechanism directly beneath the mirror, and connecting this mechanism to the mirror by a yoke, the fill factor is improved significantly.
The actuation mechanisms described in the earlier TI Patents still apply to this hidden hinge architecture and a number of hinge configurations are possible. One key drawback of this architecture is the complex fabrication process required and this manufacturing process is set forth in the Patent. Despite this drawback, the small mirror size and high fill factors required for high-resolution projection displays has forced TI to employ this method of fabrication in their commercial DMD arrays.
U.S. Pat. No. 5,233,456 issued to Nelson discloses a method to extend the basic DMD architecture to larger mirror sizes. According to this patent, a larger mirror can be supported by a plurality of torsion hinges included along the axis of rotation. These torsion hinges, and the method of fabrication, are identical to the standard DMD design. The Patent covers a resonant mirror fabricated using this approach.
U.S. Pat. No. 5,212,582, also issued to Nelson, is virtually identical in content to U.S. Pat. No. 5,233,456, except that it describes how the hidden hinge architecture can be used for fabricating large mirrors.
U.S. Pat. No. 5,312,513 issued to Florence et al. discloses a method of forming phase light modulators using the basic DMD hidden hinge architecture. These phase light modulators are of the piston type. A particularly unique feature of this invention is that the phase modulation is achieved using binary addressing circuitry, i.e., the pistons are operated in digital mode. Phase modulation is achieved by resolving light from two or more sub-elements, each designed to have different zero-offset phase differences.
In U.S. Pat. No. 6,025,951 issued on Feb. 15, 2000 to Swart et al., there is disclosed a light modulator for at least one of amplitude and phase modulation of incident light, which includes a mirror, a substrate provided with electrodes, and a hinge extending between the substrate and the mirror. The hinge is displaceable between deformed and extended positions for allowing for the displacement of the mirror. More particularly, the hinge that is provided between the SLM's substrate and the micromirror is a “flexure” hinge that comprises integral hinged sections which allow for the micromirror to tilt or to move in a piston-like fashion relative to the substrate in response to an electrostatic force. The hinge is particularly suited for supporting relatively large (>50 μm) micromirrors prone to sagging in the middle if their support is provided only at their edges, such as by conventional torsion hinges. In addition, the hinge can provide improved heat sinking capability over previously reported SLM hinge designs based on torsion bars and cantilever beams. The flexure hinge has sufficient mechanical flexibility that the SLM actuation voltage required is not excessively high. The flexure hinge has many applications, the preferred ones being a SLM for modulating high-energy beams and a SLM for optical channel switching.
FIGS. 2a to 5 and 5 to 7 illustrate respectively first and second micro-SLMs of this U.S. Pat. No. 6,025,951, which includes integrated flexure hinges.
More particularly, FIGS. 2a, 2b, 3a, 3b and 3c illustrate a micro-SLM M having a first integrated flexure hinge 40 with FIG. 2b showing the hinge 40 in an enlarged state. The hinge 40 is integrated with a mirror 42 and will be referred to as the flexure hinge 40 based on its behavioral properties and characteristics, i.e., in view of how it operates. An opening 98 is defined in the mirror 42 around the hinge 40, as seen in FIGS. 2a and 2b. Although only flexure hinge 40 is shown for supporting the mirror 42, a plurality of flexure hinges 40 can be used to support a single mirror, particularly if the mirror is large.
The flexure hinge 40 is electrically conductive so that the mirror 42 can be electrostatically actuated. The flexure hinge 40 does not have a fixed axis of rotation, as in the case of torsion hinges, for example. Nevertheless, the axis of rotation moves only slightly when the mirror 42 is actuated. An approximate position of the axis of rotation 44 of the mirror 42 is shown in FIG. 2a for illustration purposes.
In FIGS. 3a, 3b and 3c, various two-dimensional vertical cross sections of the SLM M with its electrostatically actuated mirror 42 incorporating the flexure hinge 40 are shown. More particularly, FIG. 3a shows the mirror 42 in an unbiased state, with FIG. 3b showing the mirror 42 being operated in amplitude modulation mode, whereas FIG. 3c shows the mirror being operated in phase modulation mode. The flexure hinge 40 is located on top of a substrate 46 and two address electrodes 48 and 50 are deposited on the substrate 46.
With reference to FIG. 3b, when a potential is applied between the mirror 42 and one of the address electrodes 48 and 50, in this case the electrode 48 as seen in FIG. 3b, an electrostatic force 52 causes the mirror 42 to tilt. This represents the amplitude mode of operation. A sufficiently high potential will cause the mirror 42 to tilt until it touches the substrate 46. Releasing the potential causes the mirror 42 to return to approximately the horizontal position due to restoring torque of the hinge 40.
Now referring to FIG. 3c, if equal potentials are simultaneously applied to both electrodes 48 and 50 with respect to the mirror 42, an electrostatic force 54 causes the mirror 42 to be pulled down towards the substrate 46 while substantially remaining in a horizontal position. This is the phase modulation mode of operation.
With reference to FIG. 4, the flexure hinge 40 comprises a lower horizontal web 56 sitting atop the substrate 46 and a pair of integral side arms 58 of composite configuration which extend upwardly and symmetrically from opposite edges of the web 56. Each side arm 58 includes a lower angled section 60, an intermediate substantially horizontal section 62, an upper angled section 64, and a mirror-supporting flange 80 at the upper end of the angled section 64.
The geometrical parameters which characterize the flexure hinge 40 are shown in FIG. 4 and include the thickness t of the material forming the flexure hinge 40, the length w of the web 56, the respective lengths l1, l2 and l3 of the lower angled section 60, of the horizontal section 62 and of the upper angled section 64, respectively, and a hinge width d. Furthermore, the web 56 and the lower angled section 60 define therebetween an angle θ1, with the lower angled section 60 and the horizontal section 62 defining therebetween an angle θ2, whereas the horizontal section 62 and the upper angled section 64 define therebetween an angle θ3. The restoring torque of the flexure hinge 40 is a complex function of all of these parameters. Moreover, it also depends on the mechanical properties of the hinge construction material. It must be noted that the flexure hinge 40 material does not necessarily have to be of the same thickness, or of the same type, as that of the mirror 42.
The flexure hinge 40 can be designed so that it is rigid enough to support the weight of a mirror without sagging, while at the same time being flexible enough that actuation voltages are manageable. It is reminded that a mirror, such as the mirror 42, supported with this type of flexure hinge 40 can be operated in either phase or amplitude modulation mode.
Now referring to FIGS. 5 to 7, there is shown a second flexure hinge 70 of U.S. Pat. No. 6,025,951, which is similar to the first flexure hinge 40 in how it behaves (see FIGS. 6b and 6c showing the hinge 70 subjected to electrostatic forces in amplitude modulation mode and in phase modulation mode, respectively). As with the first flexure hinge 40, the second flexure hinge 70 is electrically conductive so that the supported mirror 42 can be electrostatically actuated. The mirror 42 and its hinge 70 are sitting atop the substrate 46 and form a second SLM M′. Operation of a mirror supported by this hinge 70 is the same as that in the first hinge 40, as illustrated in FIGS. 6a, 6b and 6c which show the micro-SLM M′ respectively in an unbiased state, in amplitude modulation mode, and in phase modulation mode. Therefore, as before, the mirror 42 can be operated in either amplitude or phase modulation mode.
As for the first hinge 40, the flexure hinge 70 of FIG. 7 comprises a lower horizontal web 72 sitting atop the substrate 46 and a pair of integral side arms 74 of composite configuration which extend upwardly and symmetrically from opposite edges of the web 72. Each side arm 74 includes a lower angled section 76, an upper angled section 78 and a mirror-supporting flange 80 at the upper end of the upper angled section 78.
The geometrical parameters which characterize the flexure hinge 70 are shown in FIG. 7 and include the thickness t of the material forming the flexure hinge 70, the length w of the web 72, the respective lengths l1 and l2 of the lower angled section 76 and of the upper angled section 78, respectively, and a hinge width d. Furthermore, the web 72 and the lower angled section 76 define therebetween an angle θ1, whereas the lower angled section 76 and the upper angled section 78 define therebetween an angle θ2. The restoring torque of the flexure hinge 70 is a complex function of all of these parameters. It also depends on mechanical properties of the hinge construction material.
It is easily understood that both flexure hinges 40 and 70 of U.S. Pat. No. 6,025,951 include a pair of symmetrical composite side arms which are adapted to fold under the mirror 42 either symmetrically and identically under phase modulation mode (where the mirror 42 is drawn translationally towards the substrate 46 while remaining substantially horizontal), or individually under amplitude modulation mode (where one side arm is retracted or folded while the other is deployed).